The present invention relates to data transmission and, more particularly, a method of data transmission combining broadcast and point-to-point transmission topologies for efficient and reliable data transfer.
Emerging wireless communication systems are intended to provide voice and data communication between a wide variety of devices. Two such systems are systems complying with the SPECIFICATION OF THE BLUETOOTH SYSTEM, CORE, The Bluetooth Special Interest Group, 1999 (Bluetooth systems) and systems complying with the SHARED WIRELESS ACCESS PROTOCOL (CORDLESS ACCESS) SPECIFICATION (SWAP-CA), The HomeRF Technical Committee, 1999 (HomeRF systems). Bluetooth and HomeRF systems are short range wireless radio systems operating in the unlicensed Instrument, Scientific, and Medical (ISM) band at 2.4 GHz. Frequency hopping, spread spectrum techniques are utilized to reduce interference and fading in the noisy environment in which these systems will operate. For example, with Bluetooth systems information is exchanged as packets in a slotted communication channel. Slots in the communication channel are 625 μsec. in length and data packets are typically one-slot in length but can cover up to five slots. The system shifts transmission frequency for each slot by hopping through a psuedo-random sequence of 79 frequencies displaced from each other by 1 MHZ. The Bluetooth and Home RF systems support both asynchronous (primarily, data) and synchronous (voice) communication.
Typically, data communication between end systems of wireless networks utilizes a point-to-point communication topology. Point-to-point communication with acknowledgment by the receiver affords the opportunity to utilize error detection and control techniques to provide reliable data delivery. Reliable delivery is important for data transfer because any corruption of the data can render an entire data object useless. In the Bluetooth system forward error control (FEC) is applied to the packet header and a Cyclic Redundancy Check (CRC) is applied to the payload of data packets. FEC can also be applied to the data payload if it is determined that the communication channel is sufficiently unreliable to justify the additional overhead. The receiver acknowledges successful receipt of each packet with a reverse transmission to the sender and if an error is detected in either the packet header or the payload an automatic request for retransmission (ARQ) is issued. In response, the sender retransmits the data until the recipient either signals that the data was correctly received or a predetermined number of transmissions are exceeded. While point-to-point communication provides reliable delivery, it also requires that at least one copy of the data be transmitted individually to each recipient. Therefore, the total time required to distribute a data file to all recipients is proportional to the number of recipients and, if the number of recipients is large, the transmission time can become excessive.
On the other hand, broadcasting a single copy of the data simultaneously to all recipients eliminates the redundant transmission of copies to individual recipients that is inherent in point-to-point communication. Since the time required for data distribution by broadcasting is independent of the number of recipients, broadcasting is potentially more efficient when communicating with a large group. However, delivery is unreliable when broadcasting because the sender is unaware of the recipient's success in receiving the data. To improve the likelihood of success, data may be rebroadcast, one or more times, but rebroadcasting is performed without knowledge of the current conditions of the communication channel and the total time required for transmission increases in proportion to the number of times the data is broadcast. Since the likelihood of error increases as the size of the transmitted data object increases, broadcasting is not used to transmit large data objects in noisy, dynamic communication channels. Short range wireless systems, such as Bluetooth and HomeRF systems, are characterized by communication channels that are noisy and subject to multi-path fading due to structures in the transmission area.
What is desired, therefore, is a communication method for efficiently and reliably transferring data to a large number of recipients in a noisy communication environment.